The technical field of the invention includes methods and apparatus for monitoring, detecting, indicating, evaluating and signaling electric arcs or sparks.
The chaotic electromagnetic emanations manifesting themselves as electric arcs or sparks are closely linked to matter, wherein electromagnetic interactions bind electrons to nuclei in atoms and molecules and wherein the fundamental unit of electromagnetic radiation is the photon.
Indeed, spectra of electric arcs and sparks extend practically from DC through the entire radio-frequency spectrum and through microwave, infrared and light spectra.
Useful exploitations of the electric arc and spark phenomenon include the electric arc lamp, electric welding, the electric-arc-type of metallurgical furnace, the arc type of ion generator in satellite thrusters and for propulsion in outer space, the spark-plug-type of ignition in internal combustion engines, and electric spark ignition in gas appliances.
Unfortunately, the same quality of the electric arc or spark that led to electric lighting, electric arc welding and metallurgy, and ignition of internal combustion, has catastrophic effects in electrical faults that cause explosions or devastating fires through chaotic arcing or sparking.
By way of example, electric arc monitors would be useful in garages, automobile or motorcar repair facilities, gasoline (British xe2x80x9cpetrolxe2x80x9d) storage or dispensing facilities and in other areas where accidental electric arcing can cause disastrous explosions.
Moreover, fuses and circuit breakers are capable of preventing serious overload conditions, but they are generally ineffective to prevent electrical fires and other damage from accidental arcs and sparks which typically generate enough heat for a fire at electric current levels below the level at which the fuse will blow or the circuit breaker will trip. Reliable arc monitoring would thus be highly desirable in a large number and variety of electrical circuits.
These are, of course, only representative examples of fields where reliable arc or spark monitoring could be useful.
A major stagnating problem in this respect has been that prior-art development has run its course in its fear of false alarms. Of course, false alarms are the bane of alarm systems, as frequent occurrence of false alarms can nullify the utility of any alarm system.
Accordingly, in an effort to reduce the possibility of false alarms arising from radio broadcast and radio frequency security system signals, the arc detection system as disclosed in the International Patent Publication WO090/04278, by HAMPSHIRE, Michael John, rejects frequencies below about 160 kHz and above some 180 kHz of the arc signal signature, leaving for electrical fault detection only a narrow 20 kHz band at some 170 kHz center frequency. This, however, left a sample for arc detection that was dozens of times too small in the 100 kHz range for reliably detecting the occurrence of an arc signature while at the same time preventing the occurrence of false alarms equally reliably.
An arc detection system which avoids that drawback is apparent from PCT/US90/06113, filed Oct. 24, 1990 and published as W092/08143, by Hendry Mechanical Works, inventors HAM, Jr., Howard M., and KEENAN, James J., and in its corresponding U.S. Pat. No. 5,373,241, issued Dec. 13, 1994, and U.S. Pat. No. 5,477,150, issued Dec. 19, 1995, all herewith incorporated by reference herein for the United States of America and for all other countries where incorporation by reference is permitted. Reference should also be had to their corresponding EPO 507 782 (90917578.8) and resulting European national patents, and to their corresponding Australian Patent 656128, Canadian Patent Application 2,093,420, Chinese Patent Application 92102453.3, Japanese Patent Application 500428/91, Korean Patent Application (PCT) 701219/93, and Mexican Patent 178914 (9201530), all herewith incorporated by reference herein for all countries where incorporation by reference is permitted. That system avoids false alarms by converting instantaneous arc signature frequencies into a combination frequency from which arc-indicative signals are detected in contradistinction to extraneous narrow-band signals that could cause false alarms.
Against this background, a frequency selective arc detection system of a subsequently filed prior-art application, appears as a typical representative of the prior-art approach to arc detection. It accordingly presents a variety of approaches to arc detection that mainly look at frequencies in the upper kilohertz range, such as from 100 kHz to one megahertz. This, however, covers not only major portions of the public A.M. radio broadcast band, also known as xe2x80x9clong-wavexe2x80x9d and xe2x80x9cmedium-wavexe2x80x9d broadcast bands in some countries, but also the kind of control or security systems radio frequency band referred to in the above mentioned W090/04278 reference. Depending on location, one thus had to contend with dozens of extraneous signal interferences.
The same in essence applies to another embodiment in that prior-art proposal that suggests using a comb filter arrangement composed of four bandpass filters each of which has a 50 kHz passband, and three of which have a center frequency of 225 kHz , 525 kHz, and 825 kHz, respectively. In the A.M. broadcast and above mentioned control and security systems radio frequency band portion of that spectrum, 50 kHz samples can only represent minor fragments of the chaotic arc signature, raising the danger of false alarms from coincidental extraneous signals. This also affects the efficacy of the 55 kHz bandpass filter in that comb filter arrangement, inasmuch as that prior-art proposal continuously rotates its detection process among the four filter components of that comb filter arrangement.
A prior effort at arc detection that ventured into low frequency regions effected monitoring in various low frequency bands that were too narrow for reliable arc detection as apparent from articles by B. D. Russell et al., entitled xe2x80x9cAn Arcing Fault Detection Technique Using Low Frequency Current Componentsxe2x80x94Performance Evaluation Using Recorded Field Dataxe2x80x9d and xe2x80x9cBehaviour of Low Frequency Spectra During Arcing Fault and Switching Eventsxe2x80x9d (IEEE Transactions on Power Delivery, Vol. 3, No. 4, October 1988, pp. 1485-1500) indicating lack of success.
These developments in retrospect appear largely as a reaction to the perception of electric arcs as highly random phenomena borne out of the chaotic nature of arc signatures. This prior-art perception, however, ignores the fact that chaotic systems have a deterministic quality, and can be successfully dealt with, if one is able to discover what the underlying principles are and how they can be put to effective use.
Indeed, even chaotic electric lightning displays some self-similarity among its arboresque nocturnal discharges and within the branched configuration of its lightning bolts.
In this respect, pioneering work done by Benjamin Franklin and by Georg Christoph Lichtenberg back in the 18th Century casts a long shadow all the way to the subject invention.
In particular, Franklin through his famous kite experiment in a thunderstorm proved that lightning is an electrical phenomenon. Lichtenberg thereafter created his famous xe2x80x9cLichtenberg figuresxe2x80x9d in 1777 by dusting fine powder, such as sulfur, over insulating surfaces over which electrical discharges had taken place. Many of these Lichtenberg figures of electrical discharge resemble lightning in appearance and otherwise display a striking self-similarity in their patterns of branching lines and within such branching lines themselves. Manfred Schroeder compared this to diffusion-limited aggregation (DLA) in his book entitled xe2x80x9cFRACTALS, CHAOS, POWER LAWSxe2x80x9d (W. H. Freeman and Company, 1991), pp. 196, 197, 215 and 216. Kenneth Falconer, in his book entitled xe2x80x9cFRACTAL GEOMETRYxe2x80x9d (John Wiley and Sons, 1990), pp. 270 to 273, also applied the DLA model to electrical discharges in gas.
By way of background, fractals are phenomena in the fractal geometry conceived, named and first explained by Benoit Mandelbrot in 1975. Fractal geometry in effect is a manifestation of the fact that the natural world does not conform to an Euclidean type of geometry. Euclidean geometry is based on characteristic sizes and scaling. The natural world is not limited to specific size or scaling. Euclidean geometry suits man-made objects, but cannot realistically express natural configurations. Euclidean geometry is described by formulas, whereas the mathematical language of natural phenomena is recursive algorithms.
Such recursiveness is an expression of nature throughout destructive if not chaotic influences, manifesting itself, for instance, in a persistent invariance against changes in size and scaling, called self-similarity or self-affinity. Fractals are self-similar in that each of various small portions of a fractal represents a miniature replica of the whole. Such small portions are herein called xe2x80x9cfractal subsetsxe2x80x9d.
Electric arc or spark monitoring generally addresses itself to so-called arc signatures which are part of the electromagnetic spectrum of arcs or sparks situated in frequency bands way below light, heat radiation and microwave spectra.
Problems in this area include false alarms from mutual induction among neighboring monitored circuits. In this respect, reference may be had to a standard equation for mutual induction, such as between a monitored circuit in which an arc is occurring, and a neighboring monitored circuit in which no arc is occurring at the time:
In=2xcfx80fMIas/Znxe2x80x83xe2x80x83(1)
wherein:
Ias=arc signature current flowing in the monitored circuit where an electric arc occurs at the moment,
In=current induced by the arc signature in a monitored neighboring circuit where no arc has occurred at the moment,
M=mutual inductance,
Zn=impedance of said neighboring circuit, and
f=frequency.
As between neighboring circuits, the current In induced in a neighboring monitored circuit by current Ias flowing in the monitored circuit where an arc is occurring, decreases with decreasing frequency of that primary current Ias. However, electromagnetic arc signatures are characterized by a special shape approximating an inverse frequency (1/f) progression of their amplitude. If this is put into the above Equation (1) one gets
In=(2xcfx80fMIas/f)/Znxe2x80x83xe2x80x83(2)
in which xe2x80x9cfxe2x80x9d would cancel out, so that one gets
In=2xcfx80MIas/Znxe2x80x83xe2x80x83(3)
that is, a mutual inductance and a secondary current, In, that are independent of frequency. Such considerations have led to the prior-art conclusion that lowering the frequency of arc signature bands in which arcs are monitored would not effectively reduce cross-induction and false arc alarms therefrom.
It is a general object of the invention to provide improved electric arc monitoring systems that employ novel circuitry and/or take advantage of properties of electric arc signatures not heretofore utilized.
It is a related object of embodiments of the invention to permit reliable arc monitoring at distances from electric arcs longer and with less cross-talk or induction than heretofore.
In this respect and in general, the expression xe2x80x9cmonitoringxe2x80x9d is herein used in a broad sense, including monitoring, detecting, indicating, evaluating and/or signaling electric arcs or sparks, whereas the word xe2x80x9carcxe2x80x9d is herein used generically to cover electric arcs and sparks interchangeably as being essentially the same phenomenon.
From one aspect thereof, the subject invention exploits the discovery that electric arcs are fractal phenomena not only in the visible luminous portion of their electromagnetic radiation, as heretofore thought, but in fact are fractal phenomena all the way down to the extremely low frequency band of their electromagnetic emanation into space or along wires of the circuit where the particular arc occurs. Since all essential information that signifies xe2x80x9carcxe2x80x9d is thus contained in each fractal subset, it is sufficient for arc monitoring purposes to monitor a fractal subset of the arc""s electromagnetic emanation.
The realization according to the subject invention that the fractal nature of the arc is not limited to its visible region, but in fact extends all the way down to a few cycles per second of its signature, adds to the previously known characteristics of electric arcs at least one fundamental characteristic and at least one criterion; namely, that:
1. All essential information for effective electric arc monitoring is contained in any fractal subset of the arc signature; whereby
2. the selection of the monitoring frequency band for each purpose is liberated from prior-art constraints and can truly be the result of an optimum tradeoff in sensitivity, speed of detection, prevention or rejection of false signals, desired length of travel and mode of transmission of the arc signature from the arc to the monitoring circuit in different environments.
Pursuant to these principles, the subject invention resides in a system of monitoring an electric arc having an arc signature typified by a wideband range of frequencies of a chaotic nature in a monitored circuit, and, more specifically, resides in selecting a fractal subset of the arc signature characterized by relatively long travel along the monitored circuit, and monitoring that fractal subset of the arc signature.
The expression xe2x80x9crelativelyxe2x80x9d in this context refers to the fact that the length of possible travel of the arc signal is inversely proportional to the frequency of the arc signature. In this respect, reference may be had to the familiar algebraic equation for electric current:
I=E/[R2+(2xcfx80fLxe2x88x92xc2xdxcfx80fC)2]xc2xdxe2x80x83xe2x80x83(4)
wherein:
I=electric current,
E=voltage or potential,
R=resistance,
f=frequency,
L=inductance, and
C=capacitance of the electric circuit.
From this equation a related benefit of an embodiment of the invention can be seen; namely, that a selection of the lowest frequency or longest wavelength fractal in effect amounts to a selection of the longest survivor of the different fractals of the arc signature traveling along the monitored circuit. Up to a point, one can say that the monitored circuit itself thus performs the function of a low-pass filter for the arc detection monitor. Accordingly, embodiments of the invention permit arc monitoring at considerable distances from the occurrence of arcs in the circuit, which is useful in practice for several reasons, including the capability of surveying large circuits, and the convenience of providing central arc detection monitoring stations for several different circuits.
At any rate, at low arc signature frequencies, the possible travel of the arc signal along the monitored circuit is long, relative to higher arc signature frequencies.
It also turns out that false alarms from mutual induction among neighboring monitored circuits is lowest at low arc signature frequencies, quite contrary to what the prior art would have indicated pursuant to Equations (1) to (3) set forth above, wherein the frequency factor xe2x80x9cfxe2x80x9d in the denominator would cancel out the xe2x80x9cfxe2x80x9d in the numerator in Equation (2).
However, the fallacy of that conclusion becomes apparent if certain possible radiation effects are considered. In this respect, it is well known that xcex/2 and xcex/4 antennas constitute excellent Hertzian and Marconi-type electromagnetic radiators. The wiring in many telephone exchange, electric power supply and other installations in effect often forms such antennas at the kind of radio frequencies selected by the prior art for electric arc detection purposes. Even where the length of some wiring in an installation in insufficient to constitute a quarter-wave-length antenna, certain reactances in the circuit can provide the lumped-impedance kind of tuning or xe2x80x9cloadingxe2x80x9d that renders even relatively short conductors effective radiators.
In consequence, picked-up electromagnetic arc signatures are transmitted among neighboring circuits, resulting in false alarms, unless the segment of the arc signature picked up for monitoring is of a very low frequency (VLF) according to one aspect of the invention.
Accordingly, lower frequency fractals pursuant to embodiments of the invention induce less spurious signals through cross-induction in neighboring circuits than arc signatures having higher frequencies. Low frequency fractals more effectively avoid false alarms from mutual inductance among neighboring circuits than arc signatures at higher frequencies.
In consequence, embodiments of the invention not only permit arc monitoring at considerable distances from the occurrence of arcs in a monitored circuit, but also avoid false alarms in neighboring monitored circuits.
According to a related embodiment of the invention, the electric arc is detected from a fractal subset of the arc signature at frequencies below 30 kHz . According to The New IEEE Standard Dictionary of Electrical and Electronics Terms, Fifth Edition (The Institute of Electrical and Electronics Engineers, 1993), this is the upper limit of the very low frequency (VLF) band.
A presently preferred embodiment of the invention restricts fractal subsets from which the electric arc is detected to the ELF (extremely low frequency) band which in that IEEE Standard Dictionary is defined as extending from 3 Hz to 3 kHz .
Another embodiment of the invention restricts monitored fractals to arc signature frequencies below the voice frequency band (vf) defined in that IEEE Standard Dictionary as extending from 200 Hz to 3500 Hz.
In that vein, a further embodiment of the invention restricts monitored fractal subsets to arc signature frequencies below a first harmonic of a standard line frequency in alternating-current power supply systems.
An embodiment of the invention even selects the monitored arc signature fractal subset from a frequency band on the order of a standard line frequency in alternating-current power supply systems.
According to a related aspect of the invention, an apparatus for monitoring an electric arc having an arc signature typified by a wideband range of frequencies of a chaotic nature in a monitored circuit, comprises, in combination, an electric filter having an input coupled to that arc, having a passband corresponding to a fractal subset of the arc signature characterized by relatively long travel along the monitored circuit, and having an output for that fractal subset of arc signature. Such apparatus includes a chaotic wideband signal detector having a detector input for that fractal subset of the arc signature coupled to the output of the electric filter.
From another aspect thereof, the invention resides in a method of monitoring an electric arc having an arc signature extending over a wideband range of frequencies of a chaotic nature in a monitored circuit. The invention according to this aspect resides, more specifically, in the improvement comprising, in combination, processing portions of the arc signature in two paths out of phase with each other, and monitoring the electric arc from such out of phase portions of the arc signature.
From a related aspect thereof, the invention resides in apparatus for monitoring an electric arc having an arc signature typified by a wideband range of frequencies of a chaotic nature in a monitored circuit. The invention according to this aspect resides, more specifically, in the improvement comprising, in combination, an electric filter having an input coupled to the arc, having a passband corresponding to portions of the arc signature, and having an output for such portions of arc signature, an inverting amplifier having an input connected to the output of the electric filter, and having an amplifier output, a non-inverting amplifier having an input connected to the output of the electric filter, and having an amplifier output, and a chaotic wideband signal detector having a detector input coupled to the amplifier outputs of the inverting and non-inverting amplifiers.
From another aspect thereof, the invention resides in a method of monitoring an electric arc having an arc signature extending over a wideband range of frequencies of a chaotic nature in a monitored circuit, and, more specifically, resides in the improvement comprising, in combination, treating the arc signature as a modulated carrier having a modulation indicative of the electric arc, and monitoring the electric arc by monitoring a modulation of the modulated carrier.
From a related aspect thereof, the invention resides in apparatus for monitoring an electric arc having an arc signature typified by a wideband range of frequencies of a chaotic nature in a monitored circuit, and, more specifically, resides in the improvement comprising, in combination, a modulated carrier detector having an arc signature input and a carrier modulation output.
From a similar aspect thereof, the invention resides in apparatus for monitoring an electric arc having an arc signature typified by a wideband range of frequencies of a chaotic nature in a monitored circuit, and, more specifically, resides in the improvement comprising, in combination, combined modulated carrier detectors having arc signature inputs and a combined carrier modulation output.